Transmission control in signaling systems



H. J. FISHER TRANSMISSION CONTROL 1N SIGNALING SYSTEMS.

Filed OCC. 6, 1954 F/G. .Z

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/N VEN TOR H. J. FISHER Patented Dec. 31, 1935 UNITED STATES PATENT OFFICE TRANSMISSION CONTROL EN SIGNALING SYSTEMS Harold J. Fisher,

signor to Beil Tel rated, New York, N. York Port Washington, N. Y., asephone Laboratories, Incorpo- Y., a corporation of New Application October 6, 1934, Serial No. 747,116

6 Claims.

The invention relates to transmission control in signaling systems, and particularly to circuits for reducing the effects of interfering waves in such systems.

An object of the invention is to discriminate between electrical waves oi different characteristics, for example, between useful signal waves and interfering noise waves.

Another and a more specific object oi the invention is to improve the operation of voiceoperated devices for controlling signal transmission in a two-Way telephone transmission system subject to interfering waves such as line Voice-operated switching devices, for example,

echo Suppressors or anti-singin g devices, are often used in connection with two-way telephone systems subject to a considerable amount of noise varying in amplitude, such as the usual line noise or static. Such noise may with proper operation of the on voice energy. Noise waves seriously interfere switching devices of high amplitude may either cause false operation of the switching devices or necessitate reducing the sensitivity of the latter to such a degree as to prevent proper operation on speech Waves of low amplitude. In certain circuits of the prior art, these difficulties have been minimized by the use of circuits which discriminate between speech and noise on the basis of diiierences in energy distribution with time. An example oi such a circuit is the socalled syllable detector which operates on the syllabic variations in speech but is unresponsive to the comparatively continuous noise energy.

The present invention is in th e nature of an improvement upon voice-operated switching circuits of this type.

According to one embodiment of the invention to be described more fully hereinafter,

in an echo and singing suppressor circuit located near a terminal of a two-way radio or wire telephone system, the usual voice-operated detectors associated with the transmitting and receiving circuits are responsive to outgoing speech energy to disable the receiving circuit and render the transmitting circuit operative, and to incoming speech energy to disable the tector. of the transmitting detector is sensitivity of the latter. The receiving detector and that transmitting de- A loss pad normally inserted in the input used to reduce the sensitivity of the the transmitting detector without the loss pad are made approximately the same with respect t A syllabic detector connected to o incoming energy.

the input of the transmitting circuit is responsive to outgoing detector must be made in order to prevent clipping, be operated by echoes of the static.

of quite high sensitivity on receiving, it will received speech and This merely increases the sensitivity of the transmitting detector allowing it to be operated by the echoes which does no harm as the transmitting detector has been previously disabled by the receiving detector in response to the received speech signals.

The invention will be more clearly understood from the following detailed description thereof when read in connection drawing in which:

with the accompanying Fig. 1 shows diagrammatically the invention embodied in a singing and echo suppressor circuit for a two-way telephone system; and

Fig. 2 sho-ws in greate struction of the suppressor cally illustrated in Fig.

r detail a preferred concircuit diagrammati- 1.

The diagram of Fig. 1 is not an actual circuit iagram but rather a line indicating a transm out, a normal break in single line-layout, each ission path. In this laya path is indicated by separated arrowheads and a normal make by contacting arrowheads.

mission element is to be an arrow points from element. indicates that the path To show that a transcontrolled by a device, the device towards the An arrow directed at a make point will be disabled at that point by operation of the control device, and an arrow directed at a break point indicates that the break in the path will be eliminated by operation of the control device. A resistance element in a path indicates In Fig. l the suppress nite loss in the path. circuits of the invention are shown incorporated in a four-wire circuit forming a part of a two-way radio or wire telephone system. As shown, the four-Wire circuit comprises a one-way transmission circuit EA including the one-way amplifying device A1, for

repeating telephonic signals received two-way circuit LW in to east and a one-way from the the direction from west transmission circuit WA including the one-way amplifying device Az, for

repeating telephonie signals received from a disr tant station into the two-Way circuit LW. The two one-way circuits EA and WA may be connected in substantially each other and in ener with the two-way circui conjugate relation with gy transmitting relation t LW and a corresponding two-way circuit at the east station by any suitable means (not shown), for example, by hybrid coil transformers and associated balancing networks in the manner well-known in the art.

The two-way circuit LW may be assumed to extend towards the left eventually to a subscribers set, and the circuits WA and EA toward the right to a two-way radio or wire transmission channel. For example, the circuit EA may extend to a radio transmitter and the circuit WA to a radio receiver, or both circuits may extend to a long distance telephone terminal such as an ocean cable telephone terminal.

The transmitting circuit EA is normally' disabled, for example, by an open-circuit or a short circuit at the point I, as indicated by the separated arrowheads, and the receiving circuit WA is normally operative as indicated by the contacting arrowheads at point 2. The circuit EA is rendered operative by removing the open or short circuit at point I and the circuit WA is disabled as by producing an open-circuit or short circuit at the point 2 in response to operation of a voice-operated device 4, such as an electric discharge detecting device and mechanical switching relays controlled from its output, connected to the transmitting circuit EA in front of the point I. The voice-operated device 4 is arranged to be disabled at a point 5 in its output in response to operation of a second voice-operated device 6, similar to the device 15,' connected to the receiving circuit WA at a point west 0f the point 2, in response to received speech signals.

A loss I of optimum value, say of about eight decibels, is normally inserted in the input of the Y transmitting voice-operated device 4. The sensitivity of the receiving voice-operated device 6 and that of the transmitting voice-operated device without the loss in its input, are made approximately the same with respect to incoming energy. The sensitivity of the receiving voice-operating device 6 is set so that it will not be falsely operated by incoming static or line noise in the absence of speech transmission. A syllabic detector circuit 8 is connected to the transmitting circuit EA preferably at a point west of the point of connection of the voiceoperated device thereto. The syllabic detector circuit 8 is responsive to the outgoing speech waves in EA when the circuit is transmitting to effectively remove the loss 'i romthe input of the transmitting detector d, for example, by short-circuiting the loss 'I as indicated.

When speech signal waves are being received over the circuit WA from the distant station, the syllabic detector 8, of course, may be operated by echoes of the received speech signals transmitted into the path EA at the junction with the circuit LW, so as to remove the loss l from the input of the transmitting detector il andl thus eiectively increase the sensitivity of the latter so that it will be operated by the echoes in EA. However, false switching will not be caused by this because the receiving detector 6 has been previously Operated by the received speech to disable the detector t in its output at the point 5. Thus, it will be seen that the echoes of received speech or static will not cause false switching whether the loss pad l is in or out. The use oi the normal loss in the input of the transmitting detector and of the syllabic detector for removing the normal loss improves the operation of the switching circuit in that it effectively makes the transmitting detector more sensitive to speech signals than to steady noise by the amount of the inserted loss.

In the circuits of the prior art in which a syllabic detector circuit alone is employed to cause operation of the transmitting switching relays, a dimculty is encountered due to the fact that operation of that detector is dependent on the shape of the operating wave as well as its amplitude, whereas the operation of the receiving detector, of the type commonly used, depends principally upon the amplitude of the operating wave. Thus the relative sensitivity of the two detectors to speech or to interfering waves, or the margin against false operation'of the transmitting relays, varies continually between positive and negative values, the average Value being determined by the absolute sensitivities as adjusted by the control operator to keep the amount of false operation within tolerable limits. The above-mentioned diilculty is obviated by the arrangement of the invention as described above because similar detectors are used in the transmitting and receiving branches permitting the circuit to be operated with a constant Zero echo margin, that is, so that the two detectors have equal sensitivities as seen from the point in the receiving branch where the receiving detector is bridged. On the other hand, the protection against noise afforded by the use of the syllabic detector in the prior art circuits is retained inasmuch as the sensitivity of the transmitting detector to noise waves is normally reduced by the loss circuit in its input under control of the sylf labic detector.

A preferred embodiment of the switching circuit of the invention is illustrated in Fig. 2. As indicated, the syllabic detector circuit 3 and the transmitting detector circuit il have a common input circuit 9 including the one-way amplifying i device A3, connected across the transmitting path EA near its input. The input of the syllabic detector circuit t and the input of the transmitting detector` circuit t are coupled in substantially conjugate relation with each other and in energy transmitting relation with the common input circuit e by the hybrid coil transformer ID and associated balancing network II in well-known manner.

The syllabic detector circuit S includes a single three-electrode, high vacuum detector tube I2. The grid-cathode circuit of the detector tube I2, which includes the usual leak resistance-condenser combination i for determining the operation characteristics of the detector, is connected to a winding of the hybrid coil transformer Ii). The plate-cathode circuit of the detector tube i2 is coupled by the transformer it to the input of the low-pass filter i5 having a cut-oli at approximately 22 cycles per second. The output circuit oi the lter I5 includes in series the windings of two mechanical relays I6 and I'I connected in respectively reverse order.

The transmitting detector circuit 4 comprises the three-electrode, gas-filled trigger detector tube it? the input electrodes of which are coupled to the input circuit 9 by the windings of the hybrid coil it, and the marginal relay I9 the operating winding 2i! of which is connected across the output electrodes of the detector tube I3. The grid of the detector tube i8 is negatively biased by the, grid battery 2l, and space current is supplied to the plate of tube I8 by the plate battery 22 through the high resistance 24 shunted by the condenser 25, and the operating winding 2E) of relay I9.

The biasing winding 2S of the marginal relay 2i] is biased by current from the plate battery 22 through the large resistances 23 and 2'! in series so that the marginal relay I9 will not operate until the current through its operating winding 2S exceeds a predetermined amplitude.

One purpose oi the resistance 23 in parallel with resistances 24 and 2'! in the plate circuit of the gas-lled tube i8 is to limit the plate current of the tube to a safe value. The parallel combination of resistance 2li and condenser 25 in the plate circuit, allows the operating current to build up faster giving more positive operation of relay I9. The circuit comprising resistance 28 and condenser 29 in series, connected across the plate and cathode of detector tube i8, acts in conjunction with resistances 23 and 2li to restore the gas-filled tube I8 to thn unoperated condition after the signal Voltage at the input ceases and also provides the hang-over time for relay I9.

Connected directly across the grid and cathode of the detector tube i8 is a loss circuit or pad comprising the resistances 32 and 3i and the condenser 32 in series. The resistance 3Q in this circuit is normally short-circuited over a circuit extending from the grounded cathode of tube I8 through resistance 3i), conductor 33 and the normally closed contacts 3d and 35 of the relays I? and le, respectively, to ground.

Connected across the receiving path WA preferably near its output is the input of the receiving control circuit comprising the ampliiierdetector circuit Bt, which preferably includes a gas-nlled trigger detector tube so that it will operate quickly in response to voice signals received over the path WA, which controls the operation of the receiving relay 3i connected to its output.

The marginal relay it operates in response to operation of the gas-:filled detector tube I8 to v control the operation of the hold-over relay 38 and the switching relays SS and dil. The latter relays control the transmission eiciency of the circuits WA and EA in the manner which will be described later in connection with the description of the operation of the whole system. Also, as will be described later, the operation of the relays Iii and il in the syllabic detector circuit 8 causes the sensitivity of the transmitting detector circuit to be increased, and the operation of the relay 5l in the receiving detector circuit disables the transmitting switching relays 38, 39 and it and disables the transmitting detector circuit by grounding the input of tube lil through condenser 32.

The operation of the system of Fig. 2 is as follows:

In the absence of speech signal transmission in the system, the transmitting path EA is disabled in the output of amplifier A1, by the short circuit through the normally closed switch contacts si of the switching relay et, and the receiving path WA is operative due to the normal open condition of the switch contacts of the switch relay 3s in the short-circuiting connection across the path WA at a point in front of the connection thereto of the receiving detector circuit. The sensitivity of the receiving detector circuit including the amplier-detector 365 and the relay 37 is initially adjusted so that the relay 3'! will not be operated by the static or line noise incoming over the path WA. In the idle condition of the switching circuit, that is, when no speech signals are being transmitted or received, the input electrodes of detector tube i 8 are shunted by a circuit extending from the grid of the tube through condenser 32, resistance 3l, conductor 33, normally closed contacts 3ft of relay I'I, normally closed contacts 35 of relay le and ground to the grounded cathode of the tube. The values of the circuit elements in this normal shunt circuit are so selected that a predetermined amount of loss is effectively inserted in the path of the waves impressed on the input circuit of the tube i8. The eiiect of this normal loss is to reduce the sensitivity of the transmitting detector circuit i to the desired degree below the sensitivity desired when speech signals are being transmitted over the path EA. The reduction in sensitivity is made at least suhcient to prevent operation of the marginal relay i9 in response to the normal amount of line noise impressed on the circuit from the path EA. The sensitivity of the transmitting detector circuit 4- with a portion of this normal loss removed by the action of the syllabic detector circuit 8 in the manner which will be described, is approximately the same as that of the receiving detector circuit comprising the amplifier-detector 35 and the receiving relay 3l. The elements in the syllabic detector circuit 3 are initially adjusted so that that circuit has a high operating sensitivity in order to prevent clip-ping.

` With the circLLt in the idle condition as just described, let it be assumed that speech signals from a west subscriber are being received at the input of the four-wire circuit and are impressed upon the path EA. A portion of these speech signals will be diverted irom the path EA into the common input circuit 9 of the switching circuit, and, after amplication by the amplifying device A3 therein will be impressed by the hybrid coil transformer i@ on the input of the syllabic detector circuit S and on the input of the transmitting detector circuit The portion of the amplified speech signals impressed on the detector circuit 8 will cause operation of the detector tube i2 therein. The detected speech waves will be transmitted to the low-pass lter which will effectively suppress the frequencies above 22 cycles per second and transmit the frequencies below 22 cycles to the windings of relays I6 and El.

The speech waves impressed on the syllabic detector circuit 8 and the transmitting detector circuit 4 do not represent a long continuous wave train, but occur as short trains of waves of syllable duration. The syllables of vocal intervals occur at the rate of about two to twenty cycles per second. Therefore, in the output of the low-pass lter l5 there will be produced a current impulse of one sign at the beginning of each speech syllable and another current impulse of opposite polarity at the end of the syllable.

As stated above, the windings of the relays I6 and il in the output of the lter I5 are reversely wound so that one relay, say relay i6, will be operated by the current impulse at the beginning of the syllable and the other relay I'I will be operated on the nal impulse of the syllable. A succession of short syllables therefore will keep the armatures of the relays i 6 and I 'I in a state of vibration. The effect therefore will be that while speech waves are being substantially continuously transmitted over the path EA, the relays I6 and I'I will be operated in such manner that either the armature contact 35 or the armature contact 34 will be open. Thus, the short circuit around the resistance 38 in the loss pad in the input of the detector tube I8 Will be maintained broken during the continuous transmission of the west subscribers speech currents in response to operation of the syllabic detector circuit 8. With the short circuit removed, the loss pad in shunt of the input electrodes of the detector tube I8 will comprise the condenser 32, the resistance 3l and the resistance 3S in series. The effect of adding the additional series resistance 36 in the shunt circuit across the tube electrodes is to decrease the shunt-loss so as to make the detector tube I8 more sensitive to the impressed speech currents, while the west subscribers speech signals are being continuously transmitted over the path EA. Thus, the impressed speech signals of low amplitude as well as those of high amplitude irnpressed upon the input of the transmitting detector circuit will cause ionization to take place in the gas-lled detector tube I8 and cause the marginal relay I9 to be operated in the following manner.

The ionization of the gas in the detector tube I8 will cause the space path of the tube to become conductive to transmit space current from the plate battery 22 substantially instantaneously. The resulting output current of tube I8 will flow through the series resistances 23 and 2A and the operating winding of the marginal relay I9 and will cause operation of the latter relay to shift its armature from the back contact d3 to the front contact 44.

As soon as the armature of the marginal relay i9 leaves the back` contact 43, the normal short circuit around battery e5 through the normally closed contacts t of the hold-over relay 38, the normally closed back contact 43, relay I9 and armature of the series resistance il and the normally closed back contact 158 and armature of the receiving relay 37, is broken. This causes a surge of charging current to flow from battery 45 through the relays 39, 40 and 38 to the condenser i9 and resistances 5I) and 41 to the closed contact d8 and armature of relay 31 to ground. This surge of current causes relays S8, 39 and 40 to operate positively to attract their armatures whenever the armature and back contact d3 of relay IQ is broken.v When the armature of the marginal relay I9 reaches its front contact 44, current is supplied to the windings of the switching relays 39 and Il and the hold-over relay 38 from battery 55 over a circuit extending from ground through battery d5, the windings of relays 39, 4B and 38 in series, armature and front contact M of reay I9, series resistance l1 and the back contact i8 and armature of relay 31 to ground which holds the relays 38, 39 and 48 operated and discharges condenser @9 through the armature and contact iii of relay IS and resistance 58. Relay 39 will operate to close its normally open switch contacts #l2 applying a short circuit across the path WA which will prevent signal energy subsequently received over the path WA from the east subscriber or echoes of the west subscribers speech from reaching the receiving amplifier-detector 36 or the west sub'- scriber. Simultaneously, relay lill will operate to open switch contacts il removing the short circuit from the path EA and allowing the main portion of the speech currents of the west subscriber after amplification by the amplifying device A1 to be transmitted out over the path EA to the distant station. Simultaneously, the holdover relay 38 will operate to open its normally closed switch contacts 4B to produce another break in the normal short circuit around the battery 45 and relays 89, 4S and 38.

Now, when the west subscriber pauses for an appreciable interval of time, the amplifier signal- 5 ing voltages supplied to the input circuit of the syllabic amplifier detector circuit 8 and the input circuit of the transmitting detector circuit li through the hybrid coil transformer I8, will be removed. This will cause the detector tube I2 to lo return to the idle condition and thus cause the energizing current applied therefrom through the low pass lter I5 to the windings of the mechanical relays IS and Il to be removed. The armatures of these relays will then fall back to l5 their normal condition so as to reapply the short circuit around the resistance 3i) in the loss circuit in the input of the gas-lled detector tube I8, thus increasing the loss in the input of the latter tube and effectively reducing the sensitivity of 2o the tube to the normal value Simultaneously the signal voltage is removed from the input electrodes of the gas-lled tube I8 in the transmitting detector circuit Il. The gas-filled tube I8 will be extinguished and its 25 space current will cease to flow. However, the marginal relay I9 will remain operated for an additional hang-over interval due to the flow of charging current over the circuit extending from battery 22 through resistances 23 and 24, 30 the operating winding 253 of relay i9, to condenser 29 and resistance 28 to ground. When condenser 2e has been charged to the proper voltage the flow of current through the operating winding 28 of relay I9 will cease andthe relay will release 35 causing the armature of the relay to shift from the front contact it to the back contact 3. As soon as the armature of relay IS leaves the front contact 4, the energizing circuit for the switching relays 39 and lli! and the hold-over 40 relay 38 from battery i5 will be broken. However, the relays 39, 68 and 38 will not immediately release because their windings will be maintained energized for an additional Yhang-.over interval while the condenser et, which in series with the t5 resistance 58 formed a. shunt around the closed armature and front Contact lli of relay i9 when the latter relay was operated, is being charged by current from battery 45 through the relay windings. 50

When condenser do has been charged to the full value of the battery e, the flow of energizing current to the winding of relays 39, B and 38 will cease and the relay armatures will fall back to their normal positions. The re- 55 lease of relay 48 will cause the normal short circuit to be reapplied across path EA. The release of relay 3S will remove the short circuit from the path WA. The release of the hold-over relay 38 will return its contacts di, to the nor- 60 mal closed condition and the battery i5 will again be short-circuited through the armature and closed contact et of relay 33, the back contact 43 land armature ci marginal relay I9, the resistance /i'l and the back Contact 48 and ar- 65 mature of the receiving relay 37 to ground. Condenser 49 will then discharge through the series resistance 58 and the back Contact 43 and armature of relay le and through armature and contacts i5 of relay 33 to batter] i5 to ground. 70 The hold-over relay 3S in this way controls the hang-over time of the switching relays 39 and et. By suitable choice of the values of the elements in the hang-over circuit, the relay maybe maintained operated for a sufliciently long time interval after the west subscriber ceases talking to insure that the outgoing speech currents in the path EA have passed the short-circuiting point in that path before the short circuit is reapplied through the closing oi switch contacts lil. Also, by this means the switching relay 3S may be maintained operated for a suii'iciently long time after the supply ci signaling voltage to the transmitting detector circuit ceases to insure that the short circuit across the path WA through the closed contacts 42 o1" relay 39 will be maintained long enough to prevent all echoes of the outgoing speech currents from reaching the input oi the receiving detector circuit 6 and causing false operation thereof.

Now, let it be assumed that speech currents of an east subscriber at the distant station are being received over path WA, and, at the time of their arrival at the point of connection oi the receiving control circuit 5 to the path WA, the west subscriber has not yet started to talk.

A portion of the east subscribers speech currents will be diverted into the receiving control circuit S and will cause operation of the arnplier-detector circuit 36 therein to energize the winding oi the receiving relay Si' causing the operation of that relay to shift its armature from its back Contact 48 to its iront contact 5i. As soon as the armature leaves the back contact 48, an additional break in the energizing circuit for the transmitting relays 539 :and will be produced, so that subsequent operation of the marginal rel-ay ie cannot possibly result in operation of the transmitting relays, As soon as the armature of relay 3i reaches its iront contact 5i the resistances iii a in the loss pad in the input oi the transmitting detector tube I8 will be short-circuited over a path extending from ground through the armature and front contact 5i of relay Si, conductor 52 and the resistances 3i and 38 in series to the grounded cathode oi detector tube i8. The circuit shunted between the grid and cathode of the detector tube i3 now comprises only the condenser 32, and the sensitivity oi the detector tube l?. is so reduced that it is efiectii-'ely disabled.

Meanwhile, the main porton of the east subscribers speech currents wilbe amplified by the amplifying device A2 in the output or the path WA and the amplifying currents will be transmitted thereover to the input of the four-wire circuit and from thence over the two-wire path (not shown) to the receiving circuits of the west subscriber. The portion oi any echoes of the east subscribers speech currents which may be transmitted into the input of the path EA through the coupling at the west end of the four-wire circuit, will, oi course, be impressed on the input of the syllabi-3 detector circuit 8 and the input of the transmitting detector circuit d. over the circuit 9.

Because of the high sensitivity of the syllabic detector circuit 3 that circuit will occasionally be operated by the echo currents, in the same manner as previously described for the outgoing speech currents. This will cause operations of the relays Eil and il which tend to increase the sensitivity of the transmitting detector circuit 4 by removing the short circuit around re ance 3d, which ordinarily would pe 't operation of detector i and the relays Eil, and iii by echoes impressed upon the input oi the transmitting detector circuit 4. However, such false operations of the detector 4 and the re lays 38, 39 and 40 cannot occur as the previous operation of the receiving relay 3l, in response to the east subscribers speech currents, has disabled the energizing circuit for relays E8, 39 and i0 and short-circuited the grid-cathode circuit of gas-lled tube I8 in the manner described above. The receiving detector circuit S is arranged to have suiiiciently long hang-over time to hold the relay 3l operated for a short period after transmission of speech current in the path WA has ceased, so as to permit the echoes which may be stored in the two-wire connecting circuit to be thoroughly dissipated before enabling the transmitting branch.

Vhen the east subscriber ceases talking the receiving relay 3l releases and the elements of the switching circuit will return to their normal condition as indicated in Fig. 2. It may be desirable to produce a suitable hang-over in the operation of the receiving relay, for example, by making it slow to release by proper mechanical design, to prevent interruption of the received speech by false operation of the transmitting relays.

Many other changes which may be made in the circuits illustrated and described which would be Within the scope of the invention will occur to persons skilled in the art. The invention is only to be limited in accordance with the appended claims.

What is claimed is:

l. In combination, a source of speech waves subject to relatively steady noise waves of lower amplitude than said speech waves, a wave-operated control device connected to said source so as to be supplied with waves therefrom, switching apparatus responsive to operation of said control device, the operating sensitivity of said control device being normally reduced at least to such a degree that said device Will be unoperated by impressed waves of said lower amplitude received from said source, a second control device connected to said source so as to be supplied with waves therefrom, said second device being unresponsive to the comparativaly steady noise waves from said source but operatively responsive to the syllabic variations oi impressed speech waves of substantially all amplitudes, and means responsive to operation of said second device to eiiectively increase the operating sensitivity of the first-mentioned control device to permit quick 0peration of the latter device by impressed speech waves.

2. The combination of claim 1 in which the operating sensitivity of said first-mentioned control device is reduced by means of a loss normally inserted inthe input thereof, and in which said means responsive to operation of said second device increases the operating sensitivity of said nist-mentioned device by eiiectively removing said loss from the input thereof.

3. In a voice-operated circuit, a source of voice waves subject to comparatively steady noise waves of lower amplitude than said voice waves, a wave-operated detector circuit connected to said source so as to be supplied with waves therefrom, a switching device responsive to operation of said detector circuit, means normally inserting a loss in the input of said detector circuit at least suicient to prevent operation thereof by impressed waves oi said lower amplitude, .a second detector circuit also connected to said source so as to be supplied with waves therefrom, said second detector circuit being arranged to be operatively responsive to the syllabic variations in speech but to be unresponsive to comparatively steady noise Waves, and means responsive to operation of said second detector circuit by impressed speech waves to remove effectively said loss from the input of the iirst-mentioned detector circuit.

4. In a two-way telephone system, a four-wire circuit comprising two oppositely directed oneway paths for repeating telephonic signals in opposite directions between two two-way circuits, a Wave-operated control device connected to one one-way path and responsive to signal transmission therein to control the transmission eciency of said one-way paths, the sensitivity of said device being normally adjusted at least so that it will be unoperated by impressed noise waves of amplitude below a certain value, a second waveoperated control device connected to the other path and responsive to signal transmission therein to disable the rst device in its output, a third control device connected to said one path, which is responsive to transmitted speech waves of substantially all amplitudes in said one path but unresponsive to the comparatively steady noise waves, to increase the sensitivity of said first control device so as to permit quick operation of the latter device by impressed speech Waves.

5. The system of claim Li in which the sensitivity of said first-control device is normally adjusted by a loss of suitable value inserted in the path of the impressed waves in the input of said first device, and the sensitivity oi said rst device is increased by removing eiiectively said loss from the input of said rst device.

6. In a two-way telephone system, a four-wire circuit comprising two oppositely directed oneway paths for repeating telephonie signals in opposite directions between two two-Way lines, a wave-operated control device connected to one of said paths near its input and responsive to outgoing telephonie signals therein to disable the other one-way path near its output, a loss in the input of said device at least sufficient to prevent false operation thereof by line noise in said one path, a second wave-operated control device, connected to said other path beyond the disabling point therein and responsive to received telephonic signals to disable the first-mentioned device in its output, the sensitivity of said second device being normally adjusted so that it will be unoperated by noise below a given amplitude in said other one-way path, a third control device connected to said one one-way path near its input so as to be supplied with waves therefrom, said third control device being unresponsive to the comparatively steady noise waves in said one path but being operatively responsive to the syllabic variations in the impressed telephonic waves, and means responsive to operation of said third control device to remove the loss in the input of said iirst-mentioned device and thus effectively increase the sensitivity of the latter device.

HAROLD J. FISHER. 

